1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a power transistor capable of decreasing the on-resistance.
2. Description of the Related Art
As power transistors, a high-voltage static induction transistor (power SIT) and high-voltage junction field effect transistor (power JFET) are conventionally known, in which the current flowing between the main opposing terminals (source and drain) is controlled by a depletion layer formed by the voltage applied to the gate electrode. In these power transistors, a semiconductor layer which is located between the gate region formed in the semiconductor layer and the drain region formed in one surface of the semiconductor layer is depleted, thereby holding the breakdown voltage.
In these power transistors, however, when the semiconductor layer is depleted by turn-off, electrons flow to the power supply circuit through the drain electrode. Holes in the gate region flow to the gate circuit through the gate electrode. This gate current increases the load on the gate circuit and makes the gate circuit itself bulky.
To suppress the gate current and reduce the load on the gate circuit, a structure called a MOS-SIT has been invented (Jpn. Pat. Appln. KOKAI Publication No. 2002-83963). In MOS-SIT, a trench is formed in a substrate. An insulating film is formed on the inner surface of the trench, and then, a gate electrode material is buried in the trench. The insulating film suppresses the inflow of holes to the gate region at the time of depletion. For this reason, the gate current can be suppressed, and the load on the gate circuit can be reduced. However, when a high voltage is applied between the source and drain, a very large electric field concentrates on the edge portion on the bottom of the trench, and dielectric breakdown occurs in the insulating film.
To solve this problem, in Jpn. Pat. Appln. KOKAI Publication No. 2003-69042, a p-type impurity region is formed on the bottom of the trench filled with the gate electrode, thereby relaxing the high field concentrating on the edge portion on the bottom of the trench. This impurity region can also function to suppress the inflow of holes to the gate region, like the gate insulating film. However, the impurity concentration in the impurity region is much higher than that in the substrate. Even when the transistor is not operating, a depletion layer by a built-in potential always spontaneously extends. Hence, the current path is already narrow or is closed in the initial state, and the on-resistance is very high. For this reason, a power transistor capable of decreasing the on-resistance has been demanded.